Sophia Suarez

New Generation of Ordered Polymer Electrolytes for Lithium Batteries

Department of Physics, Hunter College of City University of New York, New York, New York 10021, USAIn poly~ethylene oxide!-based solid electrolytes, ionic conduction can occur by cations moving inside the helix~along the helixaxis! and by anions moving on its envelope. This particular mode of ion transport can be enhanced by alignment of the polymericstructural units. We describe a procedure for orienting the helices in the perpendicular direction, the result of which is a one-order-of-magnitude increase in polymer electrolyte~PE! conductivity and a similar decrease in PE/electrode interphase resistance.This procedure could also be of importance in the orientation of polymers in the nanoscale for various applications.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1803434# All rights reserved.Manuscript submitted February 27, 2004; revised manuscript received May 19, 2004. Available electronically October 6, 2004.

A Fundamental Study of the Transport Properties of Aqueous Superacid Solutions

An extensive investigation of the transport properties of aqueous acid solutions was undertaken. The acids studied were trifluoromethanesulfonic (CF(3)SO(3)H), bis(trifluoromethanesulfonyl)imide [(CF(3)SO(2))(2)NH], and para-toluenesulfonic (CH(3)C(6)H(4)SO(3)H), of which the first two are considered superacids. NMR measurements of self-diffusion coefficients (D), spin-lattice relaxation times (T(1)), and chemical shifts, in addition to ionic conductivity (sigma), viscosity (eta), and density measurements, were performed at 30 degrees C over the concentration range of 2-112 water to acid molecules. Results showed broad maxima in sigma for all three acids in the concentration range of 12-20 water to acid molecules. This coincided with minima in anion Ds and is attributed to a local molecular ordering, reduced solution dielectric permittivity, and increased ionic interactions. The location of the maxima in sigma correlates with what is observed for hydrated sulfonated perfluoropolymers such as Nafion, which gives a maximum in ionic transport when the ratio of water to acid molecules is about 15-20. Of the three acids, bis(trifluoromethanesulfonyl)imide was found to be the least dependent on hydration level. The occurrence of the anticorrelation between the ionic conductivity maximum and the anion self-diffusion minimum supports excess proton mobility in this region and may offer additional information on the strength of hydrogen bonding in aqueous media as well as on the role of high acid concentration in the Grotthuss proton transport mechanism.

Multinuclear NMR study of the effect of acid concentration on ion transport in phosphoric acid doped poly(benzimidazole) membranes.

(1)H and (31)P NMR spectra, line widths, spin-lattice relaxation times (T(1)), and (1)H self-diffusion coefficients (D) were determined for two distinct poly(benzimidazole) (PBI) proton exchange membranes (PEM), para-PBI and dihydroxy-PBI (2OH-PBI), both incorporating varying concentrations of phosphoric acid. The study was performed over the temperature range of 20-180 °C, for phosphoric acid concentrations of 30, 50, and 70 wt %. Of the two samples, less mobility was indicated for the 2OH-PBI compared with the para-PBI at all acid concentrations. It was also observed that increasing the acid content resulted in an increase in the temperature at which the T(1) minimum or plateau occurred. (31)P spectra reveal the presence of pyrophosphates and in the case of the 50 and 70 wt % para-PBI samples higher oligomers such as tripolyphosphates. (1)H D data showed the 30 wt % para-PBI having almost identical values as the 70 wt % 2OH-PBI over the entire temperature range. In general, stronger short- and long-range interactions were observed in the 2OH-PBI matrix, yielding reduced translational proton transport compared to that of para-PBI. While these stronger interactions hinder translational proton diffusion, they could enhance proton transport by the Grotthuss or structure diffusion mechanism, the more favorable transport mechanism. Activation energies obtained from the (1)H D data supports a proton-hopping mechanism, with possible assistance from fast exchange between phosphate groups.

Nuclear magnetic resonance of polymer electrolyte membrane fuel cells.

In this review, the contribution of NMR spectroscopy to the development of the proton exchange membrane fuel cell (PEMFC) is discussed, with particular emphasis on its use in the characterization of structure and transport in proton exchange membranes (PEMs). Owing to copious amount of information available, results of the past decade will be the main focal point. In addition, its use as a screening tool for the PEM materials will be discussed.

Do TFSA anions slither? Pressure exposes the role of TFSA conformational exchange in self-diffusion

Multinuclear ((1)H, (2)H, and (19)F) magnetic resonance spectroscopy techniques as functions of temperature and pressure were applied to the study of selectively deuterated 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (EMIM TFSA) ionic liquid isotopologues and related ionic liquids. For EMIM TFSA, temperature-dependent (2)H T1 data indicate stronger electric field gradients in the alkyl chain region compared to the imidazolium ring. Most significantly, the pressure dependences of the EMIM and TFSA self-diffusion coefficients revealed that the displacements of the cations and anions are independent, with diffusion of the TFSA anions being slowed much more by increasing pressure than for the EMIM cations, as shown by their respective activation volumes (28.8 ± 2.5 cm(3)/mol for TFSA vs 14.6 ± 1.3 cm(3)/mol for EMIM). Increasing pressure may lower the mobility of the TFSA anion by hindering its interconversion between trans and cis conformers, a process that is coupled to diffusion according to published molecular dynamics simulations. Measured activation volumes (ΔV(‡)) for ion self-diffusion in EMIM bis(fluoromethylsulfonyl)amide and EMIM tetrafluoroborate support this hypothesis. In addition, (2)H T1 data suggest increased ordering with increasing pressure, with two T1 regimes observed for the MD3 and D2 isotopologues between 0.1-100 and 100-250 MPa, respectively. The activation volumes for T1 were 21 and 25 cm(3)/mol (0-100 MPa) and 11 and 12 cm(3)/mol (100-250 MPa) for the MD3 and D2 isotopologues, respectively.

Cyclic phosphonium ionic liquids

Summary Ionic liquids (ILs) incorporating cyclic phosphonium cations are a novel category of materials. We report here on the synthesis and characterization of four new cyclic phosphonium bis(trifluoromethylsulfonyl)amide ILs with aliphatic and aromatic pendant groups. In addition to the syntheses of these novel materials, we report on a comparison of their properties with their ammonium congeners. These exemplars are slightly less conductive and have slightly smaller self-diffusion coefficients than their cyclic ammonium congeners.

A Study of the Effect of Heat-Treatment on the Morphology of Nafion Ionomer Dispersion for Use in the Passive Direct Methanol Fuel Cell (DMFC)

Aggregation in heat-treated Nafion ionomer dispersion and 117 membrane are investigated by 1H and 19F Nuclear Magnetic Resonance (NMR) spectra, spin-lattice relaxation time, and self-diffusion coefficient measurements. Results demonstrate that heat-treatment affects the average Nafion particle size in aqueous dispersions. Measurements on heat-treated Nafion 117 membrane show changes in the 1H isotropic chemical shift and no significant changes in ionic conductivity. Scanning electron microscopy (SEM) analysis of prepared cathode catalyst layer containing the heat-treated dispersions reveals that the surface of the electrode with the catalyst ink that has been pretreated at ca. 80 °C exhibits a compact and uniform morphology. The decrease of Nafion ionomer’s size results in better contact between catalyst particles and electrolyte, higher electrochemically active surface area, as well as significant improvement in the DMFC’s performance, as verified by electrochemical analysis and single cell evaluation.

Investigation of Dynamics in BMIM TFSA Ionic Liquid through Variable Temperature and Pressure NMR Relaxometry and Diffusometry

A comprehensive variable temperature, pressure and frequency multinuclear (1H, 2H, and 19F) magnetic resonance study was undertaken on selectively deuterated 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (BMIM TFSA) ionic liquid isotopologues. This study builds on our earlier investigation of the effects of increasing alkyl chain length on diffusion and dynamics in imidazolium-based TFSA ionic liquids. Fast field cycling 1H T1 data revealed multiple modes of motion. Through calculation of diffusion coefficient (D) values and activation energies, the low- and high-field regimes were assigned to the translational and reorientation dynamics respectively. Variable-pressure 2H T1 measurements reveal site-dependent interactions in the cation with strengths in the order MD3 > CD3 > CD2, indicating dissimilarities in the electric field gradients along the alkyl chain, with the CD2 sites having the largest gradient. Additionally, the α saturation effect in T1 vs. P was observed for all three sites, suggesting significant reduction of the short-range rapid reorientational dynamics. This reduction was also deduced from the variable pressure 1H T1 data, which showed an approach to saturation for both the methyl and butyl group terminal methyl sites. Pressure-dependent D measurements show independent motions for both cations and anions, with the cations having greater D values over the entire pressure range.